Diaphragm valves are useful in many applications. One such application for a diaphragm valve is in a gas chromatograph that is used to perform chemical analysis. Gas chromatography is useful for determining the chemical composition of various materials. It is useful for analyzing minute quantities of complex mixtures from industrial, biological, environmental, and chemical sources. Gas chromatography is also useful for preparing moderate quantities of highly purified compounds otherwise difficult to separate from the mixture in which they occur.
Briefly described, gas chromatography is the process by which the components of a mixture are separated from one another by volatizing the sample into a carrier gas stream that is passed through and over a packed column. A packed column is a tubular gas conduit filled with a bed of packing comprising a 20-200 mesh solid support. The surface of the packing is typically coated with a relatively non-volatile liquid and is known as the stationary phase. Alternatively, the column can be a capillary tube having the stationary phase material coating the inside surface of the tube with no packing material. A carrier gas, such as helium, flows continuously in this column. When a gaseous sample is introduced into an entrance end of the column, the carrier gas carries the sample to the exit, or effluent, end. Different components of the sample under analysis move through the column at different rates due to different affinity and solubility of the components in the sample with respect to the stationary phase, and so appear separately at the effluent end. The time it takes for a specific component to travel from the inlet side of the column to the effluent side of the column is a characteristic of the component.
The effluent end is the end at which the components of the sample are detected. The detection is normally achieved by measuring physical or chemical properties of the effluent. The properties measured include thermal conductivity changes, density differences, optical absorption, or ionic detection. For a sample mixture having multiple components, the detector outputs a signal that can be represented by a pulse train having multiple peaks. The position of each peak relates to the type of the components in the mixture while the area of the peak relates to the quantity of that component in the mixture.
A gas chromatograph can be logically divided into three components. They are the injector, the column, and the detector. The injector measures and delivers a precise quantity of the sample under analysis into the column. The column separates the components in the sample. The detector detects and quantifies the components in the sample.
Because the injector controls the flow of minute quantities of the sample, or gas flow, it is preferably constructed using micro-machining manufacturing techniques. Such techniques allow the injector components to be fabricated to exact dimensions. The sample quantity can be accurately controlled either by a fixed volume or a time injection technique. In fixed volume injection, a fixed volume is first filled with the sample and then injected into the column. For time injection, both the flow rate and the time the sample is allowed to pass onto the column are controlled. In either technique, a set of diaphragm valves is used to control both the carrier, or column, flow and the sample flow. They are used to block and switch direction of the sample flow.
In a conventional diaphragm valve, a circular depression is created on a flat surface. Inside the depression are two valve ports that are connected to separate external fluid connections. A diaphragm fabricated of a flexible material is positioned above the valve ports such that there is a gap between the diaphragm and the surface of the valve ports. Application of an actuating pressure to a surface of the diaphragm opposite the valve ports causes the diaphragm to deflect toward, and contact at least one of the valve ports. This position is known as the off, state of the valve because fluid communication between the ports is blocked. Removal of the actuating pressure returns the valve to its relaxed, or on state. In the on state, the flexible diaphragm returns to its relaxed position, away from the valve ports, thereby exposing the ports and allowing the sample to flow therethrough.
To improve the blockage of fluid flow in the off state, it is desirable that one of the valve ports be located in the center of the depression. It is also desirable that the centrally located valve port include a valve seat. The valve seat is an annular elevation in the well, below the major surface of the wafer in which the ports are formed, and surrounding the centrally located port. Hence, when actuated, the diaphragm will be pushed against the valve seat when in the off state. The valve seat decreases the contact surface area, thus increasing the pressure of the seal when the valve is in the off state.
The flexible diaphragm of a conventional valve can be constructed of a polyimide material such as KAPTON®, which is a registered trademark of the DuPont DeNemours company. The diaphragm may be constructed using either a single layer material, such as KAPTON® HN, VN, FPC, KN, E, EN or A, or may be constructed using a multiple layer material, such as KAPTON® FN, as will be described below.
KAPTON® is a compliant material so that it may easily deflect and seal around the valve seat upon application of the actuating pressure. The diaphragm is sandwiched between a silicon die, which includes the valve ports, and a backing glass. The diaphragm is preferably held in place via an adhesive. If the diaphragm is constructed of a single layer material, then a separate adhesive can be used to bond the diaphragm to the silicon die and the backing glass. Alternatively, the diaphragm can be clamped between the silicon die and the backing glass. Further, the valve may be constructed using other materials, such as stainless steel.
If the diaphragm is constructed of a multiple layer material, the polyimide material from which the diaphragm is constructed can be coated with material that exhibits an adhesive property when the diaphragm is bonded to the silicon die. For example, the adhesive properties may be introduced by raising the temperature of the material. Furthermore, in the case of a single layer material to which adhesive is applied, it is preferable to apply the adhesive on the surfaces of the diaphragm as a continuous thin sheet before applying the diaphragm to the silicon die, which includes the valve ports, and the backing glass.
When implemented as a multiple layer structure, several types of polyimide film are suitable for the diaphragm. While KAPTON® type FN is preferable, other materials may also be used. KAPTON® type FN comprises a composite structure having a KAPTON® type HN core with a TEFLON® FEP fluorocarbon resin on both surfaces. This fluorocarbon resin is heat sealable, and thus provides the adhesion property that is desirable on the surface of the diaphragm to bond the diaphragm to the silicon die and to the backing glass. During fabrication, the diaphragm may be bonded at elevated temperature and pressure in order to reduce fabrication time. The bonding process can be modified by adjusting the bonding parameters, which include time, temperature and pressure. Thus, bonding at a lower temperature and pressure can be achieved by increasing bonding time. Further, other materials may be used instead of KAPTON® type FN for the diaphragm. For example, any flexible material that exhibits the desired properties can be used.
When implemented as a single layer structure, an adhesive is applied to the surface of the diaphragm or the diaphragm is clamped in place.
Many applications for diaphragm valves require the valves to operate at high temperatures. Because some applications require these valves to normally be maintained in the closed, or off state, the diaphragm is pushed onto the valve seat for extended periods of time. Unfortunately, when using a diaphragm constructed of a material that adopts adhesive properties at elevated temperatures, an increase in operating temperature causes the surface of the diaphragm having the polyimide material to bond to the valve seat. Once this bonding begins to occur, the valve will not operate properly.
Therefore, it would be desirable to prevent bonding between the valve seat and the diaphragm.